Extreme marine events revealed by lagoonal sedimentary records in Ghar el Melh during the last 2500 years in the northeast of Tunisia

The Tunisian coast has been affected in the past by many events of extreme marine submersion (storms and tsunamis). A high-resolution study along two sediment cores “GEM3” and “GEM4” taken from the lagoon of the Ghar el Melh was performed to identify the different paleo-extreme events and to reconstruct the paleo-environmental changes of the Northeastern part of Tunisia during the Late Holocene. A very high-resolution of analysis (sedimentological, granulometric, and geochemical) was applied on these cores. These cores were also dated with isotopic techniques (Cs, Pbex, C) and the 15 outcomes reveal fives phases of paleoenvironmental changes of this lagoonal complex. The first phase dated from -275 to 200 Cal AD characterized by a high percentage of Silt and Clay (fine particles) indicating a protected lagoon. The second phase synchronous with the Dark Age Climatic Period dated from 200 to 1100 Cal AD, is marked by an increase in the coarse sediment and could be explained by a weakening of the sandy barrier due to an increase of storm events. The third phase is characterized by a return to a closed lagoon during the Medieval Warm Period (from 1100 to 1690 Cal AD). The fourth phase 20 dated from 1690 to 1760 Cal AD coincide with “the Little Ice Age” and is marked by one specific sedimentological layer attributed to a marine submersion event. This layer could be associated to the 1693 tsunami event in southern Italy or an increase of extreme storm events. The fifth phase covering the last 250 years present a reclosing of the lagoon.

past catastrophic events that had occurred in 1797 and 1833 (Natawidjaja et al., 2006). The Mediterranean basin is defined as an "Hotspot" of the climate change (Lionello and Scarascia, 2018 ;Pausas and Millán, 2019). According to the IPCC (2014), the coastal erosion and submersion is one of the results of an elevation of sea-level due to the global warming. The Mediterranean Sea surface temperature has increased by almost 1°C since 1980 and expected to rise further by 2.5 °C in the next seventy years (Jacqueline Karas, 2006). In this context, the Mediterranean coastal zones will 35 be probably more vulnerable to the climatic extreme events (storms and medicanes), and more exposed to coastal erosion processes and flooding (Seisdedos et al., 2013). The Mediterranean basin is also characterized by a high seismic activity due to its geographical position between the Eurasian and African plates (Papadopoulos and Fokaefs, 2005;Papadopoulos and Baskoutas, 2009). The earthquake of the 21 st of May 2003 that happened in the Western Mediterranean Sub-basin precisely in Boumerdes (Algeria) with a magnitude of about 6.8 has generated a tsunami (Belazougui, 2008). The low wave generated has 40 a height < 25 cm and propagated from the Algerian coast toward the Murcia Province. These waves have not caused any evident damages (Álvarez-Gómez and Gonzalez, 2011). However, in the Eastern Mediterranean sub-basin, the Crete earthquake (21 st July 365 AD) induced a tsunami that propagated across the region to reach Alexandria and has hit the Cretan coast causing extensive damage (François, 1984).
Due to the absence of data covering a sufficiently long time period, the recent instrumental data, and textual archive on extreme 45 events (storm and tsunami) in Tunisia do not allow us to determinate any evolution in time. According to Morton et al. (2007); Dezileau et al. (2011);Sabatier et al. (2012) and Degeai et al. (2015), these sedimentary inputs coming from the sea during marine submersions, will permit the study of these extremes events over a longer period of time. A historical catalog of tsunamis and storms is being built in the Mediterranean region (e.g., Goldfinger et al., 2013), but there is a very few information on the Tunisian coasts. The aim of this study is to reconstruct the evolution of Ghar el Melh lagoon and to identify the different 50 extreme events (storms or tsunamis) using a multi-proxy analysis based on geochemical and sedimentological approaches of two sediment cores GEM3 and GEM4 collected from the lagoon of the Ghar el Melh in the North-western part of the Gulf of Tunis.
2 The study site of Ghar El Melh

Geological Sitting 55
This work focuses on the Ghar el Melh lagoon situated in the Northeast of Tunisia. This lagoon is called also "the Porto Farina". It has an elliptic shape and approximately a surface of 28.5 km 2 . Its average depth is about ~0.8 m (Romdhane, 1985;SCET-ERI, 2000;Moussa et al., 2005). This lagoon is directly limited in the north side by a mountain range called "Jbel Nadhour" (325 m). This mountain is composed of a marine Pliocene material (Figure 1). The lagoon is bordered by marshy grounds to the west and to the south. While in the eastern side, it was separated from the sea by a sandy barrier, with a local 60 opening (El Boughaz) allowing a permanent hydraulic communication (Oueslati et al., 2006). According to Paskoff (1994), https://doi.org/10.5194/nhess-2020-311 Preprint. Discussion started: 3 November 2020 c Author(s) 2020. CC BY 4.0 License. the lagoon was considered as a vestigial and remaining part of the Utique Sea that was formed during the last postglacial transgression since 6000 years ago. Progressively, this small gulf has been disconnected from the Utique Sea, due to Medjerda fluvial deposits (Paskoff and Trousset, 1992). This caused a progressive reshaping of the lagoon to its present morphology, which could be attributed to an association of the shape of the coastline and alluvium deposition from the Medjerda River 65 (Moussa et al., 2005).

Climatological and hydrological settings
The Northeast of Tunisia is characterized by a typical Mediterranean climate with hot arid summer and rainy winter characterized generally by heavy rainfall periods and floods. The rainfall mean annual in the lower valley of the Medjerda is around 500 mm.yr-1 (Oueslati, 2004). Furthermore, in this study area, the rainfall rate is very variable. The mean winter 70 temperatures are around ~11 °C. The highest precipitation occurred mainly between November and December with an average of 248 mm (INS, 2001). The monthly average values sometimes exceed 100 mm (Beni Atta Station in December) and never fall below 60 mm (MEAT, 2001). The high temperatures are observed in August with mean values of ~27 °C. In other hand, the salinity in the lagoon depends on the hydrological balance and varies from 36 g/l in winter to 51 g/l in summer due to the highest evaporation rate 1450 mm/year (Added, 2001;Moussa et al., 2005;Oueslati et al., 2006). 75 3 Material and methods

Sampling location and sediment samples
Two piston cores were collected in 2012 in the Northeast of Ghar el Melh lagoon. These cores are 126 cm (GEM4) and 98 cm (GEM3) length ( Figure 1). They were manually sampled according a transect East West into the lagoon (∼200m from the sandy barrier for GEM3 and ∼400m for GEM4). In the laboratory, the two cores GEM4 and GEM3 were divided, 80 photographed, and described in detail. Before granulometric and geochemical analyses, the cores were split into 1 cm vertical sections. Moreover, 29 surface sediment samples were collected from the Medejerda watershed to the sandy barrier of Ghar el Melh to assess the origin of the lagoon sediment: (i) 6 samples comes from the Medjerda River (Gr01 to Gr06), (ii) 12 samples from different small affluents located in the northern and western part of the lagoon (from Gr07 to Gr18) and (iii) 11 samples from the sandy barrier (Gr19 to Gr29) ( Figure 1). For all samples, the position was registered using a GPS "GPSmap 60, 85 Garmin". In the laboratory, granulometric and geochemical analysis were performed on these surface samples.

Sedimentology and geochemistry analysis
To determinate the distribution of grain size a particle Size analysis was adopted by using a Beckman Coulter© LS 13 320 (Geosciences Montpellier). Due to the high concentration of shells fragments (>200 µm), every sample was sieved at 150 μm before analyses. An ultrasounds were used to avoid particles flocculation, after the entrance of sediments into the fluid module 90 https://doi.org/10.5194/nhess-2020-311 Preprint. Discussion started: 3 November 2020 c Author(s) 2020. CC BY 4.0 License. of the granulometric. We used an XRF core scanner Niton XL3t to analyzed sediment cores and surface samples by X-ray fluorescence (XRF). In order to avoid desiccation of the sediment and contamination of the XRF measurement unit, samples had to be covered with an Ultralean film. The geochemical analysis from XRF measurements was executed in the mining type ModCF proline mode. These semi-quantitative elemental measurements were performed along with the sediment core every 1 cm. The elemental concentrations obtained are expressed in ppm or percentage values. To evaluate the analytical error and 95 the precision of measurement, international powder standards (NIST2702 and NIST2781) were adopted, it was between 10 and 25% for the "Si", and lower than 5% for the "Ca", "Sr", "Ti", "Fe", "Zn", "Ba", and "Rb", and between 5 and 10% for the "Pb", "Zr".

Chronological framework
To determinate the chronology of the two cores GEM4 and GEM3, an accelerator Mass Spectrometry (AMS) 14 C dates on 100 only one mollusk species (Cerastoderma glaucum) associated with the using of 210 Pb and 137 Cs measurements were adopted.
In fact, the 137 Cs and 210 Pbex activities analysis was measured by gamma spectrometry using a CANBERRA Broad Energy Ge (BEGe) detector on fine sediment (fraction < 150 µm). The 210 Pbex dating is founded on the determination of the 210 Pb excess activities preserved in the sediment of cores. The utilize of this natural radionuclide 210 Pb to indicate sedimentation rate is now a well-established technique (Goldberg, 1963;Krishnaswamy et al., 1971a;Robbins and Edgington, 1975). The dating of 105 137 Cs was released following the procedure of Robbins and Edgington (1975).
The 14 C analysis was conducted at the labratoire de Mesure C14 (LMC14) on the ARTEMIS accelerator in the French CEA institute (Atomic Energy Commission) at Saclay, France. These 14 C analyses were realized with the classical procedure illustrated by Tisnérat-Laborde et al., 2001. The radiocarbon ages were transformed into calendar ages utilizing the Marine 13 curve (Reimer et al., 2009). The radiocarbon ages of marine and lagoonal organisms are generally ancient than the 110 atmospheric 14C ages and have been calculated and modified by subtracting the « reservoir age » (Zoppi et al., 2001;Siani et al., 2001;Reimer and McCormac, 2002;Sabatier et al., 2010 ;Dezileau et al., 2016).

Characterization of different detrital surface sources
To better characterize the geochemistry of sediment sources in the lagoon of Ghar el Melh, we analyzed the geochemical 115 results obtained for the sediment samples with a statistical method called principal component analysis (PCA) (Figure 2). This statistical approach was applied to distinguish the different surface sediment sources in relationship with the geochemical processes or the chemical elements proprieties. The PCA dataset input consists of 29 samples that were analyzed for their major and trace elements ("Ca", "Sr", "Fe", "Rb", "Ba", "Ti", "Mn", "Zn" and "Si") and were presented as elemental concentrations (9 variables). The two first factorial factors represented in the PCA diagram present (Figure 2) 72.25% of the 120 https://doi.org/10.5194/nhess-2020-311 Preprint. Discussion started: 3 November 2020 c Author(s) 2020. CC BY 4.0 License. entire variance of the dataset. The factor 1 accounts for 61.12% of the entire variance. Factor number 1 is marked by a positive loading for terrigenous elements "Rb", "Ti", "Ba", "Mn", "Fe", and "Zn". Whereas the "Ca" and "Sr" present a modest positive loading and are inserted in factor 1. Factor 2 represents 11.13% of the global variance. It indicates positive loading for "Si", "Mn", "Ti", "Ba", "Zn", "Rb", and "Fe", whereas "Sr" and "Ca" have negative loadings. Based on these statistical analyses of geochemical data, three distinct sources of sediments were identified ( Figure 2): (i) Terrigenous or Alluvial source 125 ("Mn", "Fe", "Zn", "Ba", "Rb") mainly discharged by rivers during floods. (ii) Marine source "Si" such as sands coming from the sandy barrier coastal during marine submersion; (iii) and a biogenic origin ("Sr" and "Ca").
According to this funding, the sedimentation in the lagoon of the Ghar el Melh is generally manifested by marine and terrestrial inputs. Indeed, the geographic distribution ( Figure  The chronology of the GEM3 and GEM4 cores has been established using the 14 C dates and also the 137 Cs and 210 Pb measurements on monospecific mollusk shell samples and bulk sediments, respectively. In the uppermost 30 cm of the two cores, the measured 210 Pbex values range in GEM3 from 287.47 to 2.29 mBqg −1 and f in GEM4 from 253.67 to 1.01 mBqg -1 . In general, the down-core distribution of 210 Pb excess values follows a relatively exponential decrease with depth. Based on a 145 model Constant Flux, Constant Sedimentation Rate (CFCS) (applicated by Goldberg, 1963;Krishnaswamy et al., 1971), the 210 Pb data determinate a sedimentation rate of 0.7 mm year −1 for GEM3 and 0.6 mm year −1 for GEM4. The distribution profile of 137 Cs activity shows for the first 10 cm ( Figure 5) a maximum value at 3.5 cm in GEM3 and 3 cm in GEM4. This can represent the period of the utmost radionuclide fallout in the Northern Hemisphere which was related to the weapons atomic peak testing in 1963. The 137 Cs obtained sedimentation rate is about ∼0.7 mm year −1 for the GEM3 which is slightly higher 150 than that obtained for GEM4 (0.6 mm year −1 ; Figure 6). The distribution profile of the total "Pb" shows that the beginning of the industrial pollution in 1892 (Latour, 2019) is situated at 9 cm, indicating a sedimentation rate of about 0.75 mm year −1 for https://doi.org/10.5194/nhess-2020-311 Preprint. Discussion started: 3 November 2020 c Author(s) 2020. CC BY 4.0 License. both cores (Figure 6). Thereby, sedimentation rate of the GEM3 core calculated using the 137 Cs, 210 Pb and total "Pb" show similar values around 0.75 mm year −1 , which are slightly higher than that for GEM4 (mean value of around of ∼0.6 mm year −1 ).
The conventional AMS-14 C measurements were performed using mollusk shells (Cerastoderma glaucum) on eight control 155 points for GEM3 (Table 1) and seven ones for the GEM4 (Table 2). Taking into account both the radiocarbon and 210 Pbex dates, the local 14 C reservoir age in the Ghar el Melh lagoon was determined. According to the method of Sabatier et al. (2010), the evaluation of the modern 14 C reservoir age was conducted by comparing an age obtained from 137 Cs and 210 Pb data, and from geochemical analysis of mining-contaminated lagoonal sediments with an AMS 14 C age of a pre-bomb mollusk shell.
According to Reimer et al. (2013), the Sea surface reservoir age R(t) for the recent shell (SacA44506) was measured by 160 subtracting the atmospheric 14 C value determined for the historical date 1845 AD (114±8 14 C years) from the measured apparent 14 C ages of the shell (450±30 14 C years, Table 3). This determines an R(t) value of 363 years. The deviance from the total mean reservoir age (ΔR) is then calculated by subtracting the marine model age value obtained for 1845 AD (453±23 14 C years) from the measured apparent 14 C age of the mollusk (450±30 14 C years; Table 3). The calculated ΔR of around 2 years (Table 3) is thus, adopted. The reservoir age of this lagoon is similar to the marine reservoir age. Finally, the age model 165 of GEM3 and GEM4 cores was established by using the OxCal 4 on 14 C ages and 210 Pbex / 137 Cs average sedimentation rates.
The 14 C mean sedimentation rate calculated is then about ∼2.5mm year −1 ( Figure 7A). As the GEM4 indicates a level of erosion or inactive deposition process named "a Condensed area" observed from 59 to 72 cm (between 0 and 1000 Cal AD/BC), which would instigate an error in the obtained age model ( Figure 7B), the discussion will be focused only on the ages estimated from GEM3 core. The age correction has been obtained by the correlation between 170 the Strontium profile of the two cores GEM3 and GEM4.

Lithological, granulometric and geochemical studies of GEM3 and GEM4 cores
Two sedimentary cores were collected in the northeastern of Ghar el Melh lagoon. They contain fine sediments (clay and silt) interbedded with coarse-grained layers formed by mollusk fragments and siliciclastic sand. The sedimentary succession in the GEM3 core is very comparable to that presented in GEM4 (Figure 8). Whereas, the thickness of sandy layers is slightly 175 different between the two cores. This could be attributed to the geographical position of core GEM3, which is closer to the sandy barrier (∼200m) than core GEM4 (∼400m).
The GEM3 and GEM4 sediment cores of 97 and 126 cm long show visual variation in the sediment composition. Lithological description of these two cores highlighted three distinct sedimentary facies (Figure 8): The Basal unit (1), situated between 126 -67 cm in GEM4 and between 97 -85 cm in GEM3, is composed generally of a lighter grey silty layer and shells. At the 180 base (the last 3 cm for the GEM3 and the last 13 cm for the GEM4) the lithological composition of this unit is characterized by a very thin fine sandy layer. For the GEM3, the transition between unit (1) to unit (2) is defined by a sharp contact; The Middle Unit (2), situated between 85 -63 cm in GEM3 and between 66 -60 cm in GEM4, is typically composed of a lighter https://doi.org/10.5194/nhess-2020-311 Preprint. Discussion started: 3 November 2020 c Author(s) 2020. CC BY 4.0 License. grey sandy layer with a combination of shell fragments and sands; The Upper unit (3): <60 cm in GEM4 and <63 cm in GEM3,is composed mainly of organic-rich clay and shells with the intercalation of a sandy layer situated between 36 and 26 185 cm in GEM3 and between 30 and 24 cm in GEM4 (Figure 8). This sand layer is usually characterized by coarse sediments with light colours and also dominated by shell fragments. This coarse grain size layer intercalated in the mud sediments indicates an "energetic" event, relative to the background sedimentation. It is probably linked to washover event and marine incursion during an intense event such as a storm or tsunami.
The granulometric analysis shows that the silty clay is the most abundant fraction in our sediment cores (Figure 8). However,190 sand is dominant at two levels. The first layer occurred between 86 and 64 cm in GEM3 and between 60 and 65 cm in GEM4 whereas the second one is observed from 35 to 26 cm in GEM3 and from 30 to 24 cm in GEM4. These two layers are characterized by the dominance of a sandy quartz material and shell fragments.
The geochemical analysis of GEM3 and GEM4 cores performed using an XRF core scanner, have detected 21 chemical elements ("Zr", "Si", "Sr", "Rb", "Th", "Pb", "Zn", "Cu", "Ni", "Fe", "Mn", "V", "Ti", "Sc", "Ca", "K", "S", "Ba", "Cs", 195 "Te", and "Sn") above the limit of detection. Among these elements, we choose to represent in Figure 8 only those with a significant down core variation which are "Si", "Ca", "Sr", and "Ti". The Silicon "Si" will represent the marine geochemical pole, while "Ti" represents the terrigenous one. According to the granulometric and geochemical results of surface samples, the "Ti" is associated with the silt-clay fraction whereas the "Si" is associated with the sandy fraction. Furthermore, the highest Strontium "Sr" and Calcium "Ca" values in the two cores are found to be related with the silt-clay fraction between 62 and 33 200 cm in GEM3 and between 60 and 30 cm in GEM4. The "Sr" and "Ca" represent a biogenic geochemical pole linked to marine shells. These results are in agreement with those acquired by the statistical method (PCA) in surface samples.

Discussion
According to our granulometric, geochemical results, we can divide the Late Holocene lagoonal history into five phases that record the connection between the lagoon and the sea in relation to the sandy barriers evolution. The different phases can be 205 described as follow: The first phase, dated from -275 to 200 Cal AD, is marked by a high percentage of fine sediments (about 70 % of Silt) ( Figure   9). Our geochemical results show a relatively high concentration of terrigenous elements "Ti" (around 250 ppm) and "Fe" (around 2500 ppm). This predominance of the fine fraction, rich in Ti and Fe, means that the lagoon is filled in sediments coming from the Medjerda River. The presence of fine material suggests that the hydrodynamic current in the lagoon is low, 210 we have a protected lagoon with a sandy barrier well-constructed. This accumulation of fine sediment could also indicates a higher contribution of sediments from Medjerda River. Indeed, this interval corresponds to a more humid phase (Roman Climatic Optimum) characterized in Garaet el Ichkel (North of Tunisia) by a stabilization phase of vegetation (Stevenson et al., 1993). https://doi.org/10.5194/nhess-2020-311 Preprint. Discussion started: 3 November 2020 c Author(s) 2020. CC BY 4.0 License.
The second phase has started at 200 Cal AD and finished at 1100 Cal AD. Our granulometric and geochemical results 215 demonstrate that the sedimentation of Ghar el Melh lagoon during this period was mainly controlled by a marine contribution. This phase is marked by a decrease in the percentage of silt, and a rise in the percentage of sand (about 80 %). This time interval shows also the presence of a high amount of Silicon (Si >100000 ppm) (Figure 9) which stipulate an enhance in the sandy material supply. The decrease in the silty-clay fraction (10%) is can be related to a dilution by high marine inputs. The dominance of coarse sediment, rich in Si, could be explained by a weakening of the sandy barrier due to an increase of storm 220 events. Degeai et al. (2015) and Sabatier et al. (2012) have clearly recorded a period of higher storm activity from 400 to 800 cal yr AD in the occidental part of the Mediterranean area. This period named Dark Age Cold Period coincides with the North Atlantic cooling phase known as Bond Event 1 (Bond et al., 2001). Dezileau et al. (2011) andSabatier et al. (2012) have demonstrated that intervals of an enhance in storm activity in the Western Mediterranean area seem to be well related to the cold periods of the Holocene. During these cold periods, the sea ice was prolonged over the North Atlantic basin southward 225 especially during winter (Lamb, 1995). Dezileau et al. (2011Dezileau et al. ( , 2016 hypothesized that this extends propose a rise in thermal gradient, leading to an enhanced lower tropospheric baroclinicity over a large Central European-Mediterranean domain. This mechanism related to a southward movement of storm track suggests an enhance of storm activity in the Western Mediterranean Sea, in accordance with simulation (Raible et al., 2007).
The third phase, dated from 1100 to 1690 Cal AD, is marked by a decrease of sandy material while the highest percentage of 230 clay (about 30%) (Figure 9) of the whole record are observed. This kind of sedimentation is typically associated with processes of decantation in the lagoon (Liu and Fearn, 2000;Donnelly et al., 2004). Geochemical results determinate an enhance in the concentration of terrigenous elements "Ti" (around 400 ppm) and "Fe" (around 3500 ppm) (Figure 9) during this interval. This increase in the silty-clay fraction is probably due to a decrease of storm activity and/or an increase of fine sediments transported into the lagoon by rivers during periods of higher runoff. During this third phase, we have a protected lagoon again with a 235 sandy barrier well-constructed.
The fourth phase has started at 1690 Cal AD and finished at 1760 Cal AD. This phase was characterized by a high deposition of sand, about 75 % of the total sediment (Figure 9), indicating an opening of the lagoon (Figure 10). This time interval was characterized by a sudden decrease of "Sr" (around 200 ppm) and "Ca" (around 55000 ppm) concentrations (Figure 9). The presence of coarse sediments can be explained by a higher marine influence. Interestingly, this coarse-grained layer is recorded 240 in both cores (from 26 to 35 cm for GEM3 and from 24 to 30 cm for GEM4) collected at 200 m and 400 m from the sandy barrier respectively (Figure 10). Do we have had a major event? The relationship between the intensity of a sea extreme event (storm or tsunami) and the grain size of the overwash sand accumulated in the lagoon is very complex (Dezileau et al., 2011).
Whereas, a positive relationship has been proved between storm-surge height, storm intensity, and the grain size of sands. Donnelly et al. (2001) revealed that outstandingly higher storm surges has been caused by historic and recent major hurricanes 245 over the Atlantic coast than winter storms and minor hurricanes and demonstrated during the past several centuries, that only the major hurricanes left a regionally consistent and stratigraphically distinct record of overwash sand layers in the deposits of https://doi.org/10.5194/nhess-2020-311 Preprint. Discussion started: 3 November 2020 c Author(s) 2020. CC BY 4.0 License. the coastal marshes. Thus, this period could be a sign of an enhance of intense storm activity. Indeed, this interval corresponds to the historic period called "Little Ice Age" (LIA) which coincides with the North Atlantic cooling phase known as Bond Event 0 (Bond et al., 2001). (Dezileau et al., 2011)); (Sabatier et al., 2012)); (Dezileau et al., 2016)) and (Degeai et al., 2015)) 250 have clearly recorded a period of higher storm activity between 1400 and 1800 cal yr AD in the occidental region of the Mediterranean Sea. This sand deposit could also be associated to a tsunami deposit, a tsunami causing a marine submersion and an opening of the sandy barrier. This period is in line with the documented Sicilian tsunami that occurred in 1693 CE.
Due to its geographic position nearby to Sicilian coast, we thought that probably, Tunisian coast and therefore Ghar el Melh lagoon can be sensitive to the 1693 tsunami. The sedimentary characteristics of tsunamis or storms deposits are almost similar 255 ((Hawkes et al., 2007); Morton et al., 2007;Kortekaas and Dawson, 2007;Mamo et al., 2009). Regarding the diagnostic characteristics of storm or tsunami deposits, several studies have pointed out many hypotheses and determinate that their distinctions are very controversial (Tappin, 2007;Engel and Brückner, 2011;Sakuna-Schwartz et al., 2015). Considering the available data, the hypothesis of tsunami or storm origin for this recorded extreme event remains open. Deciphering the origin of this event requires further investigations, such a modeling of tsunami wave propagation for example. 260 The fifth phase dated from 1760 to 2012 Cal AD, is characterized by fine sediments (Figure 9). This fifth phase shows a protected lagoon ( Figure 10) and suggesting that no catastrophic intense sea events have struck Gar El Melh lagoon during this last phase.

Conclusion
To identify the paleo-extreme events (storms or tsunamis) and to reconstruct the paleo-evolution of the lagoon of the Ghar el 265 Melh, in the Northeast of Tunisia, high resolution of sedimentological, geochemical and geochronological analyses were used.
This approach gives information about the paleoenvironmental changes of Ghar el Melh lagoon and paleo-extreme events since 2300 years. Five phases have been identified: The first phase (from -275 to 200 Cal AD) indicates a protected lagoon.
The second phase dated from 200 to 1100 Cal AD shows an opening lagoon and more marine inputs. This period was may be associated with a more storm activity during the Dark Age Cold Period. The third phase indicates an isolated lagoon and 270 coincides with the MWP (from 1100 to 1690 Cal AD). The fourth phase "the LIA" dated from 1690 to 1760 Cal AD and shows an opening of the lagoon again may be due to an enhance of storm activity or the occurrence of the 1693 tsunami event. The fifth phase covered the last 250 years and shows a reclosing lagoon. Even if our records allowed to detect past extreme events, it is not possible to differentiate if they are due to tsunamis or storms. Deciphering the origin of these events require further investigations. 275 Author contributions All authors analyzed the results and prepared the manuscript. https://doi.org/10.5194/nhess-2020-311 Preprint. Discussion started: 3 November 2020 c Author(s) 2020. CC BY 4.0 License.     The OxCal 4.3 have been used to calculated the age model with 7 samples for 14C dates and only one sample for each Pb and Cs dates. The blue band determines a period of inactive deposition process or erosion so-called "a Condensed area". https://doi.org/10.5194/nhess-2020-311 Preprint. Discussion started: 3 November 2020 c Author(s) 2020. CC BY 4.0 License. Figure 9: Evolution of grain size population compared to the results obtained from the XRF records (Iron, Calcium, Silicon, Titanium and Strontium) distributing in the GEM3 core, vs. age from -300 till 2012 AD. The two blue bands indicate a period of the Lagoon opening.